1. Field of the Invention
The present invention pertains to microfabricated structures and, more particularly, to the formation of above-substrate micro-fluidic structures, such as cavities, enclosed chambers, and channels, preferably utilizing a single-type material.
2. Description of the Related Art
Micro-ElectroMechanical Systems (MEMS) refers to the fabrication and utilization of microscopic mechanical elements, such as sensors, actuators, and electronics, typically fabricated on or in silicon chips or a silicon substrate using microfabrication technology. This technology is borrowed from fabrication techniques used to form integrated circuits (e.g., CMOS, bipolar, or BICMOS processes). MEMS devices are generally mechanical components ranging in size from a micrometer (a millionth of a meter) to a millimeter (a thousandth of a meter), and can include three-dimensional lithographic features employing various geometries.
Typical applications for MEMS devices and systems include piezoelectrics for printers or bubble ejection of ink, accelerometers to control the deployment of airbags, gyroscopes for dynamic stability control, pressure sensors used in transportation and medical applications, such as car tire pressure sensors and disposable blood pressure sensors, micromirrors used to form displays, optical switching technology for data communications, and heated chambers for fluidic applications.
A related technology is Nano-ElectroMechanical Systems (NEMS), which are similar to MEMS but on a smaller scale, including displacements and forces at the molecular and atomic scales. Together NEMS and nanotechnology have made it possible to provide mechanical and electrical devices on a single chip that are much smaller, more functional and reliable, and produced at a fraction of the cost of conventional macroscale elements. In many of these applications, chambers and channels are used for transporting, storing, manipulating, and sensing fluids both in gaseous and liquid form. The formation of these chambers and channels in MEMS devices presents unique fabrication challenges.
Today, most fluidic chambers and channels in MEMS applications are constructed from thick deposited materials in which the chamber or channel is formed by either patterning and etching or by formation in the substrate materials, such as the silicone substrate used to form integrated electronic circuits.
One of the basic building blocks in MEMS microfabrication is the use of thin-film deposition processes on a substrate, applying a patterned mask on top of the deposited film by photolithographic imaging, and etching the film utilizing a selective mask process.
Typical materials used are organic polymers, silicon, or various glass-like films. Generally, the bottom, sides, and top surrounding the channels are formed of three different material types for ease of construction. Using fewer types of material increases the difficulty of fabrication. Of the many available materials, the easiest to build with, organic polymers, have dimension control limitations because of the large shrinkage factor during curing (typically 25%). If not fully cured, they have poor adhesion characteristics and are not as resistant to the stresses of temperature and chemicals. While other materials are available with more desirable characteristics, they are impractical because of the thickness required. In some cases, tens of microns of vertical dimension are necessary in order to fabricate a fluidic chamber or channel.
FIG. 1 illustrates one type of conventional chamber structure 20 formed on a substrate 22. In this example, an optional integrated circuit 24 is formed on top of the substrate. A lower portion 26 of the chamber 28 is formed of a thin deposited film, while the chamber sidewalls 30 are typically a thick organic “spin-on” material, such as polyimide, SU8 and Fox. The top 32 of the chamber can be a rigid plate applied after the channel is formed or a deposited material applied before the channel is formed. The disadvantage of this construction is, as alluded to above, the use of three separate materials, the thin film for the lower portion 26, the spin-on material for the sidewalls 30, and the rigid material for the top plate 32. In addition to the aforementioned problems caused by the different materials, different processes are required, increasing the complexity and cost of this structure.
FIG. 2 shows another approach to forming a channel or a chamber structure 34 in which the substrate 36 is etched or otherwise excavated to form the channel 38 that is then enclosed by a subsequent layer 40. In this case, only two materials are used, but the disadvantage remains of using materials having potentially conflicting properties as well as the necessity of using different processes.